when i run some graphics-intensive game (like doom3 or farcry) just after i started my computer after couple of hours of standby, i get a descent gameplay (50-60fps) for about two minutes maximum.
then framerate drops to 15-20 or even 7-8. restarting the game and/or computer didn't give any results, framerate stays low.

only if i turn off the computer and let it "cool down" for couple of hours, i then get 50-60 fps again but for only one or two minutes. then the fps drops down again.